1. Field of the Invention
The present invention relates to a method for maintaining, when reduced metal is produced from metal oxide using a rotary hearth furnace, the hearth surface clean by removing stuck substances stuck on the hearth to prevent reduced metal discharge means (device) from being worn.
2. Description of the Related Art
With recent trend of activation of producing steel by an electric furnace, a demand for reduced iron is now increasing due to pressure for supply and demand of scraps as main raw material thereof or a demand for production of high-class steel by an electric furnace.
As one of processes of producing reduced iron, attention has been paid to a process of mixing powdery iron ore and carbon material such as powdery coal or coke to form lump-like substances, for example, pellets, charging the pellets into a rotary hearth furnace, heating it at a high temperature to thereby reduce oxide iron in the iron ore to obtain solid metal.
One example of a reduced iron producing process by a conventional rotary hearth furnace will be explained with reference to a plan view explaining a schematic constitution of equipment of the rotary hearth furnace used in the past shown in FIG. 4.
Powdery iron oxide and powdery carbonaceous material are mixed and granulated to produce raw pellets. and granulated to produce raw pellets.
The raw pellets are heated to a temperature level to a degree that inflammable volatile substance generated from pellets is not fired to remove stuck moisture to form dry pellets (raw material 9).
The dry pellets (raw material 9) are supplied into a rotary hearth furnace 7 using a suitable charging device 3 to form a pellet layer on the rotary hearth 1.
The pellet layer is radiation heated by combustion by burners 7c installed at the upper part within the furnace to reduce it, which is metallized to obtain reduced iron.
The reduced iron is cooled by directly spraying gas on the reduced iron using a cooler 8 or cooled indirectly by a water cooling jacket to discharge it outside.
In the reduced iron producing process by the rotary hearth furnace, when the lump-like substance is placed on the rotary hearth furnace, the lump-like substance is powdered to generate powder due to the mechanical shock or the like. Further, even after being placed, the substance is exposed to a high temperature atmosphere within the furnace, CO or CO2 gases are generated by de-volatilizing of volatile components in carbon or reducing reaction to increase internal pressure of lump-like substance so that the lump-like substance is broken, or explosion occurs to generate powder. The thus generated powder is reduced within the rotary hearth furnace to form powder of oxide iron.
Further, the discharge of the lump-like substance (reduced iron) which is reduced within the rotary hearth furnace and metallized is normally carried out using a screw type discharge device, but also in this case, reduced iron receives mechanical handling to generate powder.
The thus generated powder is difficult to be removed completely by the discharge device, a part of which remains on the hearth or is rubbed into the hearth surface by the discharge device. When powder is stayed in the furnace, the powder are sintered together at high temperature and stuck on the hearth, and new powder is accumulated on the stuck substance and grows. Powder contains not only metal iron but also ore components (slag components) derived from gangue in iron oxide or ash in carbonaceous substance, and the ore components (slag components) are repetitively molten and coagulated on the hearth. Further, the slag components are compressed and rolled together with metal iron by the discharge device whereby the metal iron and the slag components are mixed to create a texture having high hardness.
The discharge device is cooled by a suitable method in order to secure its mechanical strength, but since the stuck substance on the hearth is hard and at high temperature, the knife edge of the discharge device rises in temperature and becomes worn due to friction heat generated when in contact with the stuck substance. Therefore, the operation is often interrupted and work for replacing a screw of the discharge device is necessary, posing a serious problem of lowering of availability factor and higher maintenance costs.
In order to cope with the above problem, various proposals have been made to scheme a cooling method (construction) of a screw blade of the discharge device so as to reduce wear of the knife edge.
For example, one method is that a blade is made to be hollow, into which cooling water flows to thereby cool the blade, thus reducing a damage resulting from corrosion of the blade.
A further method is that a cooling pipe is arranged so as to surround the discharge device, the blade is cooled by radiation cooling thereof to reduce a damage resulting from corrosion of the blade or wear of the knife edge.
However, even if the blade is water-cooled, a temperature of the knife edge in contact with the hearth which is high in temperature and is high in hardness rises to provide less effect in reducing the wear of the knife edge. Moreover, when the wear of the knife edge progresses and cooling water leaks, the product, reduced iron becomes oxidized again.
Further, since the indirect cooling method by way of radiation cooling by a water cooling pipe is used, there is no problem of a leakage of cooling water caused by the wear of die knife edge as described above, but because of the indirect cooling, the effect of cooling the knife edge is smaller than that of the above-described invention failing to rarely provide effect for the wear of the knife edge.
In view of the foregoing, development of a method capable of easily removing the stuck substance even if the stuck substance is formed on the hearth has been demanded in order to solve the aforementioned problem basically. For example, the following proposal has been made, which is however not enough to solve the problem.
There are proposed a method of, in order to remove powder of metal iron or stuck substances staying on the hearth, blowing it off with jet gas to recover it by a suction hood, and a method of scraping it by a scraper. However, the method of blowing off by a flow of jet gas has a problem that removal of the stuck substance stuck on the hearth strongly is difficult, and powder of metal iron blown off becomes adhered within the suction hood. Further, in the method of sweeping out by a broom with a rotary blade, removal of the stuck substance stuck on the hearth strongly is likewise difficult. Furthermore, the method of scraping by a scraper has a problem that as described above, powder of metal iron is struck by the scraper and compressed and rolled, resulting in higher possibility of accelerating the growth of the stuck substances.
It is therefore an object of the present invention to provide an operating method of a rotary hearth type reducing furnace for positively removing stuck substances stuck on the hearth by a simple method to thereby prevent or reduce wear of a knife edge of a screw of a discharge device to enable continuous operation for a long period and to enable achievement of high availability factor.
An operating method of a rotary hearth type reducing furnace, an operating method of a rotary hearth type reducing furnace for producing reduced metal from raw material comprising at least metal oxide and carbonaceous material, the method including the steps of quenching a hearth surface of said rotary hearth type reducing furnace, and removing a stuck substance from said hearth.
An operating method of a rotary hearth type reducing furnace, an operating method of a rotary hearth type reducing furnace for producing reduced metal from raw material comprising at least metal oxide and carbonaceous material, the method including the steps of applying mechanical shock to a hearth surface, and removing a stuck substance from said hearth.
An operating method of a rotary hearth type reducing furnace, an operating method of a rotary hearth type reducing furnace for heating and reducing raw material comprising at least metal oxide and carbonaceous material to produce reduced metal, the method including the steps of quenching a hearth surface of said rotary hearth type reducing furnace, applying mechanical shock to the hearth surface, and removing a stuck substance from said hearth.
The operating method of a rotary hearth type reducing furnace, wherein said quenching is a directly quenching with water.
The operating method of a rotary hearth type reducing furnace, wherein a quantity of said water is changed to thereby adjust a thickness of the stuck substance removed.
The operating method of a rotary hearth type reducing furnace, wherein stuck substance breaking means installed on a ceiling part above said hearth surface is dropped on said hearth surface to thereby apply mechanical shock.
The operating method of a rotary hearth type reducing furnace, wherein stuck substance breaking means installed on a ceiling part above said hearth surface is driven up and down to thereby apply mechanical shock.
The operating method of a rotary hearth type reducing furnace, wherein after said quenching and or after application of said mechanical shock, said hearth surface is heated again before said stuck substance is removed from said hearth.
The operating method of a rotary hearth type reducing furnace, wherein said quenching position and or position applying said mechanical shock are or is the hearth surface between a position for discharging said reduced metal from said rotary hearth type reducing furnace and a position for charging said raw material into said rotary hearth furnace.
An operating method of a rotary hearth type reducing furnace for heating and reducing raw material comprising at least powdery metal oxide and powdery carbonaceous material to produce reduced metal, wherein a hearth surface of said rotary hearth type reducing furnace is quenched to generate crack in a stuck substance stuck on said hearth, after which said stuck substance is removed from said hearth.
An operating method of a rotary hearth type reducing furnace for heating and reducing raw material comprising at least powdery metal oxide and powdery carbonaceous material to produce reduced metal, wherein mechanical shock is applied to a hearth surface of said rotary hearth type reducing furnace to generate crack in a stuck substance stuck on said hearth, after which said stuck substance is removed from said hearth.
An operating method of a rotary hearth type reducing furnace for heating and reducing raw material comprising at least powdery metal oxide and powdery carbonaceous material to produce reduced metal, wherein a hearth surface of said rotary hearth type reducing furnace is quenched to generate crack in a stuck substance stuck on said hearth, and mechanical shock is applied to said hearth furnace, after which said stuck substance is removed from said hearth.